When machining a part with a cutter tool, the cutter tool becomes progressively worn. FIG. 1A shows an example of a cutter tool 10 mounted on a machining tool 20 for machining a part 30. Specifically, the part 30 is a body of revolution and it rotates about its axis, while the machining tool moves with feed movement Dx along the direction of said axis in order to travel along the entire part 30. FIG. 1B is a perspective view of the part 30 and of the cutter tool 10 seen looking along direction B in FIG. 1A. Relative to the cutter tool 10, the part 30 thus follows a helical path that is characterized by a cutting speed Vc and a feed fz per revolution per tooth (helical pitch, if f is the overall feed per revolution and Z is the number of teeth, then f=fz×Z). Another machining parameter that is often used is the depth of cut (depth of material removed during cutting).
In general manner, when machining a part with a cutter tool, material is removed by the combination of relative movements between the part and the cutter tool: the cutting movement Dc in the direction c along which the cutting force Fc acts, and the feed movement Dx in the direction x along which the feed force Fx acts. The radial force Fr acts in a direction r perpendicular to the cutting direction c and to the feed direction x. These movements and forces are shown in FIGS. 1A and 1B. In the case of turning, i.e. when the part for machining is substantially a body of revolution rotating about its axis, as in the example of FIGS. 1A and 1B, the cutting direction c and the feed direction x correspond respectively to a tangential direction and to an axial direction, with the direction r being a radial direction.
In the example shown, the cutter tool 10 has a cutting edge 16, a face 14 (cut face), and a flank 12. The face 14 is the face along which the swarf 32 resulting from cutting the part 30 is removed. The flank 12 is the face situated facing the machined surface of the part 30. The cutting edge 16 lies at the intersection between the face 14 and the flank 12. Inserting the cutting edge 16 into the part 30 creates swarf 32.
As can be seen in FIG. 2, which shows the flank 12 of the cutter tool 10 seen looking along direction II in FIG. 1A, the flank 12 has a wear zone 12a that appears and grows as the tool 10 is being used. The wear of the flank 12 may be due essentially to the high mechanical stresses that result from the part 30 rubbing against the cutter tool 10. The wear zone 12a on the flank is generally in the form of a shiny striped strip parallel to the cutting edge.
With reference to FIG. 2, an abscissa axis is defined by the feed direction x and parallel to the initial cutting edge 16, i.e. parallel to the cutting edge 16 before it has been subjected to any wear. An ordinate axis y is also defined that is perpendicular to the feed direction x in the plane of the flank 12. The wear zone 12a of the flank 12 may be characterized at each abscissa value x, by a characteristic length written VB(x) (or more simply VB) and referred to as flank wear. Flank wear VB is generally measured in millimeters (mm). As shown in FIG. 2, the flank wear VB(x) is the distance, at a given abscissa value x, between the initial cutting edge 16 and the point of the wear zone 12a that is furthest from the initial cutting edge 16, measured along the direction of the ordinate axis.
Instead of the characteristic length VB, it is known to characterize the wear of the flank by a characteristic length VBmax referred to as the maximum flank wear and calculated as being the maximum of the flank wear VB(x) over all of the abscissa values x. In order to avoid edge effects, the maximum flank wear may be calculated over only a central portion of the wear zone 12a of the flank. The extent of said central portion may be determined using criteria that are standardized and known to the person skilled in the art.
When the wear of a cutter tool reaches a level that is excessive, the cutter tool can deform, crack, or indeed flake, thereby causing scraps to become inserted in the machined part. Furthermore, damage to a tool leads to an increase in forces and in vibration during machining, which is harmful for the quality of the machined part, i.e. for its geometrical and dimensional characteristics, for its surface state, and for the integrity of its material. It is therefore essential to monitor the cutter tool and to discard it when its wear, and in particular its flank wear, becomes excessive and exceeds a certain threshold, referred to as the discard criterion.
In order to determine whether the flank wear of a tool is excessive, a method is known of monitoring flank wear on the basis of at least one other magnitude (referred to below as the “observed magnitude A”). Such magnitudes (or physical quantities) may include a power or a force exerted by the cutter tool on the part. When the observed magnitude exceeds a threshold As, machining is stopped and the cutter tool needs to be changed. The threshold As needs to be determined beforehand, during a predetermining step, as a function of the parameters of the machining.
An example of such a method is shown diagrammatically in FIG. 3, in which curves 101, 102, and 103 represent the variation in an observed magnitude A as a function of machining time t during three respective successive machining operations. As can be seen on curve 101, the observed magnitude A increases steeply at the beginning of a first machining operation (stage in which the cutter tool penetrates into the part), and then increases more slowly, and finally decreases when the tool is removed from the part and the first machining operation comes to an end. The curve 102 shows a second machining operation having substantially the same appearance as the curve 101 representing the first operation, except that the observed magnitude A reaches higher values as a result of the wear of the cutter tool. During a third machining operation, the values reached by the observed value A (which varies as represented by curve 103) are even higher, such that during the third operation, the observed magnitude A reaches the predetermined threshold As and the third operation is stopped at time t=t1.
A major drawback of such a method is the time required by the steps for predetermining the threshold As for the observed magnitude A. Specifically, insofar as the discard criterion is often expressed relative to flank wear, determining a threshold As requires correspondence to be established empirically between flank wear and the observed magnitude A, which requires a large number of machining operations.
Furthermore, although the flank wear of a tool is a magnitude that is intrinsic to the tool, the observed magnitude A, such as a force or a power, generally depends on the part being machined, and in particular on its material, on the cutting speed, and on all of the machining parameters. With such a method, a threshold thus needs to be predetermined in full once again for each machining operation and for each change of a parameter.
In addition, when the observed magnitude A has reached the predetermined threshold As, the tool is changed, but the value of the flank wear of that tool is still not known. For example, a tool may have suffered premature wear, such that the observed magnitude reaches its threshold long after the flank wear has reached a discard criterion. Under such circumstances, since the tool has been used beyond its discard criterion, the quality of machined parts may not satisfy fabrication specifications.
There therefore exists a need for a new type of method for determining the wear of a cutter tool.